It could be possible that your relative may
have been of Native American heritage. During those times, Mulatto (according to whoever was doing the census)could mean anyone with African blood. Sometimes the people who did not have any African heritage was labeled Mulatto. As for this Haplogroup being found in a certain region of the US, well we must remember groups of people traveled to different areas, intermarried, became slaves during wars, etc. So, the sky is the limit with possibilities.